Mad, bad and sad: that's artistic types for you, if star biopics are anything to go by. I'm Not There, Todd Haynes's new film about Bob Dylan, perpetuates the cliche faithfully, if idiosyncratically. A succession of actors, among them Cate Blanchett, portray the whiny, mercurial singer at various stages of his development. Minor emotions are ramped up to epic scale, as if, like some biblical hero, Dylan must transcend each artistic challenge before advancing to the next. Here, it seems, is a deeply important man whose soul-contortions are more meaningful, whose crises are more pertinent and whose perceptions are just that bit sharper than the rest. Having been groped by the Muse, Dylan has achieved exalted status here on earth and is now ready to be idolised.

And so it goes. Gifted chaps are depicted as rugged individualists powered by pure genius, no matter how seedy their actual fates or how boring their real lives. So we have Jackson Pollock played by a wiry, gurning Ed Harris, Truman Capote as a grand swell who ventured tragically out of his depth and Picasso rendered by Anthony Hopkins as a lusty old goat. In life as in film the Great Men attract subservient women and are forgiven for their unkindness because of the marvellous gifts they present to the world. Biopics from Amadeus to Velvet Goldmine fabricate their mythic importance, turning artists' demons into something sexy and compelling. Even the most immature act - say, chucking a telly out of a hotel window - becomes an Event, rank arrogance passed off as defiant self-assertion.

Even an artist's failings become reflections of greatness. As another recent biopic, Control (about the Joy Division singer Ian Curtis), showed, a star's brightness eats up the people who live at its edges: parents, old friends, failed rivals, discarded lovers. The director, Anton Corbijn, depicts the music world as a male club in which females are geishas or matrons. It is in this capacity that Samantha Morton gives one of her brazen, wounded performances as Curtis's marginalised wife. Upon her falls the burden of his "artistic personality". Yet Curtis, played by Sam Riley with melting eyes, comes out of it looking like a tragic hero, his shoddiness as a husband and father, his cowardice and hypocrisy, all excused by his talent.

In women it's the opposite: their troubles are a function of their shrill pettiness and lack of staying power. In High Art, the character based on Nan Goldin is sacrificed to heroin abuse; Jennifer Jason Leigh's seedy, squinting Dorothy Parker drinks her talent away. Female geniuses are passed off as neurotic nut-jobs and called by their first names - Sylvia, Iris, Jane, Jackie, Frida - like pet dogs. Sometimes the demeaning rewriting strays into outright falsity: we know from Hermione Lee's bestselling biography that Virginia Woolf was a speedy, twitchy type, prone to losing weight - miles away from Nicole Kidman's depiction in The Hours.

Viewers are in thrall to the Romantic inspiration that makes artists snatch up a pen and create a great work in one go, usually after some easily dissolved writer's block. In reality, the creative life is much more like Last Days, in which Michael Pitt plays Kurt Cobain, pottering ineffectively while grooming his death wish. For all their rebel mythology, artists are conservative, devotedly serving their own vision for decades. Spending so much time alone with their ego or surrounded by cronies and handmaidens, they're usually unpleasant. They're attractive only when directors, screenwriters and cinematographers labour to make them so, knowing that audiences want their delusions vindicated.